Bilayer Graphene Structure Could Lead to Better Transistors

Imagine trying to fill up a glass of water, and the more you pour in, the emptier the glass gets. It sounds far-fetched, but if you try this same experiment with electrons instead of water, it’s actually quite possible.

For scientists, the counter-intuitive behavior of electrons prompted an important question: Is there a level in a sea of electrons, and can one measure it?

The answer is yes, according to a team of researchers in the Cockrell School of Engineering at The University of Texas at Austin. Led by graduate student Kayoung Lee and associate professor Emanuel Tutuc, both in the Department of Electrical and Computer Engineering, the researchers developed a novel device structure that can measure the level of electrons in graphene bilayers. Discovered in 2004, graphene is a super strong, single-atom-thick carbon material, with properties that make it a promising material for electronics.

Gathering a basic scientific understanding of how electrons interact in graphene could help researchers in their quest to build a better, low-power transistor — the building block of electronics that acts as an on/off switch, either allowing or preventing current to flow through. Today, graphene transistors lack the ability to be turned off. A transistor that operates at room temperature is the ultimate goal as it could help solve some of electronics’ chief problems, including power consumption and over-heating.

An illustration of the researchers' bilayer graphene structure.

The researchers’ device, called a double bilayer graphene heterostructure, is a stack of two graphene bilayers separated by boron nitride, an electrical insulator. The double bilayer graphene provides unique insight into the quantum physics in this material, and could one day serve as the basis for a low-power transistor. The team’s paper, titled “Chemical potential and quantum Hall ferromagnetism in bilayer graphene,” was featured on the cover of this year’s July 4 issue of Science.

Electrical current can flow in each layer as well as between the two layers. The researchers observed that each one of the two bilayers can be used as a “gate” that allows electrons to flow in and out of the opposite layer. More importantly, the structure can also allow scientists to measure the energy of the added electrons. This is particularly important for electrons because their electrical charge, either positive or negative, conspires to dramatically change their energy.

“This study introduces a graphene-based heterostructure with potential technological ramifications,” said Sanjay Banerjee, director of UT Austin’s South West Academy of Nanoelectronics (SWAN) and Cockrell School electrical and computer engineering professor. “If you have a new kind of transistor that could operate at room temperature, it would make a major impact.”

This study was funded by the Office of Naval Research, and the Nanoelectronics Research Initiative.